This invention relates to bioremediation of contaminated vapors and, in particular, to a rotary biofilter for removing and destroying volatile organic compounds and inorganic air toxins.
Microbes, e.g. bacteria, fungi, protozoa, and yeast, are the oldest forms of life but, until recently, have been little used by man except for making bread, wine, and cheese. Only within the last twenty years have microbes been used for recycling industrial waste, principally in Europe and Japan.
Bioremediation has been used for eliminating odors and pollutants from the by-products of industry, e.g. food processing and waste water treatment. Recycling depends upon the discovery that, among the millions of omnipresent microbes, some microbes can digest waste, even substances such as cyanides that are toxic to human beings and to most other life forms.
A typical biofilter includes a filter bed, a pump for blowing untreated air or gases through the filter bed, and a humidifier connected between the pump and the filter. The humidifier is needed to prevent the gases from drying the filter bed. The filter bed is at least one meter deep and can be soil, compost, or an inorganic material having a large surface area such as sand, porous clay, or small polystyrene spheres.
The operation of a biofilter is not completely understood but is generally believed to take place in a thin film of water, known as a "biofilm," surrounding each particle in the filter bed. Gases dissolve into the film and microbes in the film use the dissolved gases as sources of carbon and energy, metabolizing or converting the gases into non-toxic by-products, such as elemental gases or carbon dioxide.
Industrial uses of biofilters typically involve large volumes of gases having a low concentration of organic pollutants, e.g., less than 1,000 parts per million. Biofiltration for pollution control can involve much higher concentrations of gases and gases that are slowly biodegradable. For example, alcohols, ethers, and some of the more common monocyclic aromatics degrade quickly. Highly chlorinated organics tend to degrade much more slowly.
The size of the biofilter depends on the volume of gases, their concentration, and transit time. Transit time is the time for the gases to flow from a supply pipe through the bulk of the biofilter to an outer surface. For complete conversion of the gases, the transit time must exceed the time required for the gases biodegrade.
In a typical application, the filter bed is large, occupying several thousand square feet of land or floor space, and is immovable. In addition to the sheer size of typical biofilters, there are two problems relating to the filter material. One problem is compaction of the filter material and the other problem is fissuring in the filter material. While seemingly opposite, both problems can occur simultaneously.
The filter bed is preferably a homogeneous, porous material having a large surface area. The water film adds weight to the material, softening it, and gravity and decomposition of organic matter causes the material to compact. As the filter compacts, it becomes less efficient at treating the gases and less permeable to the gases, i.e. the pressure drop across the filter increases and the flow of gases through the filter decreases. Compaction can also cause inhomogeneities in the filter bed because the microbes are cut off from their food source and die, decreasing the capacity of the biofilter. Stirring or raking the filter bed may not be sufficient to assure homogeneity if a die-off occurred. Fissures, on the other hand, are favored paths taken by the gases without coming into intimate contact with the biofilm. The result is untreated gases escaping from the filter. Thus, maintenance of a filter bed is a continuing problem.
Large scale industrial operations are not the only applications for biofilters. A frequently occurring example is the remediation of soils contaminated with volatile organic compounds, such as gasoline, from leaking underground storage tanks. In theory, some soils will recover on their own if the leaking is stopped but recovery can take a long time and the leakage plume may reach underground water supplies in the meantime.
Corrective action requires reducing the concentration of the pollutants as quickly as possible. Typical remedial operations include extracting the volatile compounds through a system of pipes in the ground for withdrawing the compounds from the ground and pumping the compounds into an incinerator. There are several disadvantages to incineration. The energy required for burning the contaminated vapors can be expensive, as can the maintenance of the incinerator. Also, combustion byproducts may themselves pose environmental and health risks.
For these reasons, bioremediation is particularly appealing. Capital and operating costs are lower than for other air pollution control technologies and the only emissions or by-products of a properly run biofilter are carbon dioxide, water, and biomass. However, until now, bioremediation systems typically require large areas and have not been particularly suited to temporary location at sites of small spills or leaks such as at gas stations.
In view of the foregoing, it is therefore an object of the invention to provide a rotary biofilter for converting waste or pollutants into harmless by-products.
Another object of the invention is to provide a biofilter in which compaction of the filter bed is minimized.
A further object of the invention is to provide a biofilter in which fissures in the filter bed are minimized.
Another object of the invention is to provide a biofilter which can be transported to the site of a small spill or leak of waste or contamination.
A further object of the invention is to provide a biofilter having reduced pressure drop.
Another object of the invention is to provide a biofilter requiring little maintenance.